1. Field of the Invention
The present invention relates to a private automatic branch exchange.
2. Description of the Related Art
Conventionally, a private automatic branch exchange allowing use of digital key telephone sets, analog key telephone sets, and standard telephone sets as extension terminal devices has a function of automatically judging what kind of telephone set is connected to each extension. To be concrete, a conventional extension interface device judges automatically whether a telephone set using two pairs of conductors per extension has been connected. The conventional extension interface device has a function of automatically forming a judgment no matter what kind of telephone set has been connected to an extension.
Some conventional private automatic exchanges allow parallel connection of an analog key telephone set and a standard telephone set to each extension port or parallel connection of a digital key telephone set and a standard telephone set to each extension port so that they may be used as so-called parent and child telephone sets.
In the above described conventional system, however, both the analog key telephone set and the standard telephone set connected to one extension port in parallel have the same telephone number and use the same speech path in common, and hence they are controlled as one extension. The same may be said of parallel connection of a digital key telephone set and a standard telephone set.
In such a form that a key telephone set is used by a person different from a person using a standard telephone set, therefore, problems are posed. For example, privacy cannot be ensured, or two telephone sets cannot be used simultaneously.